The present invention relates to a developing apparatus of an electrophotographic type image forming apparatus in which one-component developer composed of nonmagnetic toner particles is used to develop electrostatic latent images.
There is provided a conventional developing apparatus used for an electrophotographic image forming apparatus in which one-component developer composed of nonmagnetic toner particles is used.
In this developing apparatus, toner particles are held on a rough surface of a rotatably supported cylindrical developing sleeve so as to convey the toner particles to a developing area for development. This type of developing apparatus is applied to an image forming apparatus in which non-contact development is conducted. However, in this type of developing apparatus, relatively rough non-magnetic toner particles, the average particle size of which is approximately 10 .mu.m, are generally used for developer. Therefore, delicate lines and contrast can not be accurately reproduced by this developing apparatus. For this reason, this developing apparatus is not suitable for image formation of high quality.
In order to improve image quality in this non-contact development, it is necessary to reduce the size of toner particles so as to provide minute particles. However, when the average size of toner particles is reduced to a value not more than 10 .mu.m, the following problems may be encountered:
(1) A formed image is affected by van der Waals force compared with Coulomb's force in development. Therefore, a phenomenon of fog appears in which toner particles are deposited on a background of the image. In this case, it is difficult to prevent the occurrence of fog even when a DC bias voltage is impressed upon a developing sleeve.
(2) It becomes difficult to control a triboelectric charging operation. Therefore, coagulation of toner particles tends to occur.
(3) Fluidity of toner particles tends to change in accordance with the environment and the condition of use. Accordingly, the image characteristics tend to change in accordance with the change in the fluidity.
For these reasons, improvements in image quality provided when the size of toner particles is reduced are not so effective, and it is difficult to obtain clear images.
In order to solve the aforesaid problems, various developing methods have been proposed until now, which will be described as follows.
(1) Japanese Patent Publication Open to Public Inspection No. 27158/1981
A plurality of corona discharge wires, which are parallel with each other, are provided between an image retainer and a developing sleeve which are not contacted, and the polarities of adjoining wires are set to be reverse. Under the above condition, an AC voltage is impressed so that developer particles are scattered between the image retainer and the developing sleeve.
(2) Japanese Patent Publication Open to Public Inspection No. 198470/1982
A grid is provided between a latent image holding surface and a toner holding surface, and a bias voltage including DC and AC components, or including one of DC and AC components, is impressed between the grid and the toner holding surface.
(3) Japanese Patent Publication Open to Public Inspection No. 131878/1991
There are provided an electrode made of a plate-shaped elastic body, the end portion of which is located in a developing area so as to be contacted with developer on the sleeve, and a voltage impressing means for impressing a variable electric field between the electrode and the sleeve. By the action of the variable electric field, developer is scattered in the developing area so as to develop a latent image.
(4) Japanese Patent Publication Open to Public Inspection No. 130569/1990
In a developing method in which a bias voltage including an AC component is impressed, when the impression of the AC component is stopped, an electrical field to pull the developer particles back to the sleeve side is impressed exceeding a predetermined intensity.
(5) Japanese Patent Publication Open to Public Inspection No. 115264/1992
There is provided a multicolor image developing method in which a plurality of developing apparatus disclosed in Japanese Patent Publication Open to Public Inspection No. 131878/1991 are arranged.
(6) Japanese Patent Publication Open to Public Inspection No. 56977/1992
In the developing method disclosed in Japanese Patent Publication Open to Public Inspection No. 131878/1991, when a developing operation is not conducted, a releasing electrical field is impressed so that the developer particles deposited on the electrode can be released toward the image retainer.
However, the aforesaid developing apparatus and methods of the prior art have various problems.
By the methods of items (1) and (2), development is conducted while an alternating electrical field is impressed by a plurality of grid wires provided between the image retainer and the sleeve in a developing area. In this case, scattered developer particles clog between the wires, so that the developing properties are deteriorated. Also, it is difficult to accurately dispose a plurality of wires in a small gap of a substantial apparatus.
In the method described in item (3), an electrode to scatter developer particles also exists on an upstream side of the contact portion with the sleeve. Accordingly, when developer particles are conveyed, a variable electrical field is formed in the same manner as that of the developing area, so that the developer particles are further returned to the upstream side. As a result, an amount of developer conveyed through the electrode is excessively lowered.
In the methods described in items (4) and (5), in a multicolor image forming apparatus having a plurality of developing apparatus in which the formed toner image is transferred by one operation, fogging toner deposited on the image forming body and toner deposited on a non-image portion of the image forming body tend to be reversely conveyed to the developing apparatus since the electrical potential on the image retainer surface is high.
In the method described in item (6), in a multicolor image forming apparatus in which development is conducted a plurality of times and the formed image is transferred by one operation, toner clouds formed by toner particles released by the releasing electrical field drift in the developing space even when development operations of other colors are conducted. Therefore, mixing of color tends to occur on the image retainer especially when a continuous copying operation is carried out. Moreover, two types of power sources for impressing a voltage upon an electrode, one is for developing and the other is for releasing, are required, so that the structure of the apparatus becomes complicated.
Further, the present invention relates to a method for manufacturing an electrode used for a developing apparatus, and also relates to the developing apparatus. More particularly, the present invention relates to improvements in a control electrode used for a developing apparatus in which toner particles in developer are vibrated so as to be scattered, and the scattered toner particles are deposited on an image retainer drum.
Conventionally, there has been provided a developing apparatus in which toner particles in developer are vibrated so as to be scattered, and the scattered toner particles are deposited on an image retainer drum. In this developing apparatus, powdery toner is generally used, and a cloud is formed of the powdery toner. For this reason, this method is referred to as "a powder cloud developing method".
According to this method, minute toner particles can be used, so that a resolving power higher than that of a conventional method can be provided. As described in items (1), (2) and (3), the following proposal has been made recently: A control wire or a control electrode is attached into a developing region so that a toner cloud is electrically formed to develop toner images by means of cloud-development.